1. Field of the Invention
The present invention relates to a film element for absorbing tensile forces.
2. The Prior Art
Film elements are frequently used to transfer tensile forces in the direction of a longitudinal extent, e.g., as functional parts (in particular pressure-sensitive, i.e., self-stick) for carrying, connecting, sealing and/or covering objects. Examples from everyday life include adhesive tape and/or adhesive strips. Furthermore, a wide variety of loops, belts and hangers, shopping bags and/or shopping bags handles, load securing films (for example, over loaded pallets), sealing labels, signet labels and packaging films.
So-called hanger labels are used in the field of medicine in particular but also for pharmaceutical products. These hanger labels have a hanger-like hanging strap, which can be pulled out of the plane of the film of the remaining label, which is adhesively attached to a container, e.g., an infusion bottle. The container is suspended upside down by the hanging strap. The hanger-like hanging strap is then subjected to considerable tensile stresses, depending on the size and filling of the container.
Publications describing such hanger labels include German Patent DE 39 07 862 A1, German Utility Model DE 91 01 464 U1, European Patent EP 0 356 574 A2 and European Patent EP 0 632 422 A1, for example.
When a tensile stress acts on a film, microtears occur, starting in particular from minor defects in the material. At the end of tearing, locally elevated stresses occur, leading to propagation of the tear and ultimately to failure of the material, i.e., the film is torn all the way through.
Since the contours of hanger labels are usually produced by punching, it is possible for microtears to develop here starting from edge defects, which may result from poor punching results, e.g., due to worn a punch. These problems are even greater in rotary punching, which is considerably more productive than roller flat punching or planishing.
When the hanging strap of a hanger label is torn through, the container suspended by the hanger label falls down. In the case of an infusion bottle, this could have serious consequences for the patient being treated.
According to the state of the art, film parts which are acted upon by tension are designed with certain safety reserves by using thicker or more tear-resistant material, which is usually also much more expensive.
For example, various olefin films are used, which, although they have a greater tear propagation resistance and are thus less susceptible to damage, also have a greater elongation and a lower tensile strength, which is why very thick films (film thickness approximately 150–250 μm) must be used. Furthermore, multilayer film composites are also used. For example, a laminate of two layers of PET 75 μm and 50 μm thick will provide a greater security against damage than a single layer of PET with a thickness of 125 μm. Nevertheless, minor damage in critical regions of a hanger label, e.g., at the suspension point or at curved punched holes at the transition from a hanging strap to the glued regions of the label, may result in a significant loss of tensile strength.
Table 1 shows as an example the weakening of the hanger of a two-layer hanger label made of PET due to damage to the edge. This lists the tensile strength in newtons for the undamaged label and when there is slight damage to the edge of one of the layers.
TABLE 1Tensile strength of a two-layer PET hangerdamage toDamage toundamagedone layertwo layersTensile strength (N)206501.4
As Table 1 clearly shows, a slight damage at the edge of one layer already means a considerable susceptibility to tearing with the associated reduction in tensile strength, while damage to the edge of both layers causes a reduction in tensile strength by several orders of magnitude.
Table 2 gives the tensile strength in newtons for the damaged and undamaged condition of 25-:m-wide single-layer and two-layer film strips made of different materials. In the case of materials “A” and “B,” these are plastic materials having a lower stretchability and a greater tensile strength, whereas materials “C” and “D” are plastic materials having a greater stretchability and lower tensile strength. The :m values give the particular thickness of the film and/or the film layers.
TABLE 2Tensile strength of 25 :m wide film stripsMaterialTensile strengthTensile strength(N) undamaged(N) damaged“A” (125 μm)665less than 300“A” (75 μm)437189“A” (50 μm)252112“A” (75 μm/50 μm)556246 (both layers damaged)“B” (40 μm/40 μm)199 63“C” (101 μm) 40 34“D” (165 μm)236167“D” (60 μm) 77 56
It is easy to see from Table 2 that with traditional film strips which are acted upon by a tensile stress, a great reduction in tensile strength is evidently unavoidable whenever there is damage to the edge.